Alveoli: The Microscopic Air Sacs Powering Gas Exchange

The lungs contain ~300–500 million alveoli—microscopic, thin-walled sacs where gas exchange occurs.

Structure-Function Design

• Wall thickness: 0.1–0.5 µm (type I pneumocytes) — minimal diffusion distance.
• Surface area: ~70–100 m² total (tennis court) — massive area for exchange.
• Capillary network: Dense mesh enveloping each alveolus; blood-gas barrier ~0.5 µm.
• Surfactant: Type II pneumocytes secrete phospholipid-protein complex ↓ surface tension → prevents collapse (atelectasis).
• Pores of Kohn: Inter-alveolar connections equalize pressure, allow collateral ventilation.

Gas Exchange Physics

• O₂: alveolar air (100 mmHg) → capillary blood (40 mmHg) → binds Hb.
• CO₂: venous blood (45 mmHg) → alveolar air (40 mmHg) → exhaled.
• Diffusion-limited (CO, fibrosis) vs. perfusion-limited (O₂ normally).

Clinical Relevance

• Emphysema: destroys walls → ↓ surface area, ↑ dead space.
• ARDS: surfactant loss, edema → shunt.
• Pulmonary fibrosis: thickened barrier → diffusion limitation.

Alveolar integrity is the foundation of respiratory life.